It is generally known that exposed photographic materials which contain a gelatin/silver halide layer which is slightly hardened or unhardened can be hardened imagewise by development using tanning developing substances. The oxidation products of the tanning developer formed in the exposed areas of the image have the property of hardening gelatin. On the other hand, in the unexposed areas of the image where there is no development, this hardening does not occur. These areas of the image, therefore, can be washed off with warm water or can be transferred wholly or partially onto an image-receiving material.
It is known from U.S. Pat. No. 2,596,756 "Photomechanical Copy Method" to add pigments, especially carbon black, to the light-sensitive layers of these materials to bestow upon the transferred image parts a sufficient optical density.
A wash-off material is known from U.S. Pat. No. 3,364,024, "Photographic Process" where a silver halide emulsion layer and a carbon black-containing gelatin layer which is free of silver halide is arranged on the same side of the emulsion support. This layer containing carbon black is likewise imagewise hardened during development and contributes to the formation of the necessary optical density, thereby reducing the amount of silver halide needed to produce an image. The possibility of saving silver is of considerable significance in view of high silver prices. However, these carbon black-containing materials have considerable disadvantages which result from a reciprocal effect or action between carbon black and gelatin. Carbon black brings about an undesirable hardening of the gelatin contained in the emulsion layer on the emulsion support side and thus after a short holding time this leads to a hardening of the total layer. This undesirable hardening leads to the result that, after exposure and development, it is not possible to make a sufficient differentiation between the hardened and nonhardened areas of the image. Because this disadvantageous effect of carbon black is proportional to its concentration in the layer there is naturally a limitation to the amount of carbon black which can be incorporated in these layers and, thus, the possibility of reducing the silver halide amount required to attain a certain optical density.
In order to be able to use higher carbon black concentrations in the layers, it has already been suggested to use layers with a high content of gelatin or to introduce the carbon black in a separate layer and, as known from British Pat. No. 1,294,355, additionally to provide a separating layer between the emulsion support and the light-sensitive layer. Aside from the fact that the problem of poor storability cannot satisfactorily be solved by these measures, the resolving power of such materials is so poor because of the high layer density or the intermediate layers that they are unusable for many applications.
The object of the present invention, therefore, is to provide a light-sensitive, carbon black-containing material for tanning development which has an excellent storability and high resolving power.